Rapid development in both commuication technology and computer technology has led to the requirement of higher speed circuits, more sensitive circuits and higher gain circuits. Such requirements have arisen at the same time as the further development of integrated circuit technology in which high packing densities in integrated circuits has become highly desirable.
Optical communication technology provides an illustrative example. Optical commuication has been developing at a rapid rate during the last few years. High sensitivity in receivers is desirable because it increases the distance between repeaters and reduces the number of repeaters in a particular communication system. Also of importance in optical communication systems is high speed amplifiers which permit high bit rates to be transmitted. High speed amplifier systems are also of use in other types of circuits including logic circuits, memory circuits, analog switching, high-input-impedance amplifiers, integrated circuits, etc.
One of the most promising semiconductor compounds for high speed, high gain devices is indium gallium arsenide and related compounds such as indium gallium arsenide phosphide. This is due to the high mobility and peak electron velocity exhibited by this III-V semiconductor compound. A particular difficulty is in fabricating suitable devices to take advantage of these properties. For example, metal-semiconductor field effect transistors (MESFETs) have extremely low barrier heights between the indium gallium arsenide and the contact metal which precludes fabrication of high-sensitivity field effect transistors by this procedure. Various corrective measures can be used to improve the barrier height characteristics but a reliable process to produce such devices with reproducible characteristics has not been found. For example, barrier heights can be enhanced by interposing a thin layer of insulator material or wide band-gap semiconductor between metal and channel layer but such devices are very difficult to fabricate and small process variations result in unwanted variations in performance.
Junction field effect transistors provide another device configuration in which high electron mobility materials could be used effectively to make high speed devices. Such devices have been discussed in a reference by Y. G. Chai and R. Yeats entitled "In.sub.0.53 Ga.sub.0.47 As Submicrometer FET's Grown by MBE," IEEE Electron Device Letters, Vol. EDL-4, No. 7, July 1983, pages 252-254.
An important characteristic of field effect transistors is the transconductance of such devices, usually defined as the partial derivative of the drain-to-source current with respect to the gate-to-source voltage. Short gate lengths and close proximity of gate-to-drain and scurce electrodes are imperative for good performance of the junction field effect transistor.